Reliability analysis can be defined as the analysis of events that contribute to the occurrence of undesirable conditions, and the application of probability theory to determine that the likelihood of these undesirable conditions falls within acceptable limits. In other words, reliability analysis provides a statistical basis for making decisions as to the relative safety or usefulness of a particular device or system.
Reliability analysis is especially important in complex systems such as flight control systems in aircraft as failure of a particular component or subsystem could cause the destruction of an entire aircraft or failure of a mission. As aircraft have become more sophisticated, control systems have become more vital to the operation of these aircraft. The use of digital systems and redundancy management schemes to satisfy flight control system requirements of high performance aircraft has increased both the number of implementation alternatives and the overall system design complexity. Consequently, a comprehensive reliability analysis of each candidate architecture becomes tedious, time-consuming and costly. Current methods for reliability analysis are discussed generally in a paper entitled "Methods for Evaluating Integrated Airframe/Propulsion Control System Architectures," Cohen, Lee and Palumbo, NAECON 87, vol. 2 (May 1987), pp. 569-575.
Currently, evaluation tools exist to aid in the analysis process. Given system reliability models such as fault trees or Markov Models, these tools quantify system attributes such as mean time between failures and component vulnerabilities for flight safety, or some other reliability condition.
To define the reliability model that serves as input to an evaluation tool, a failure mode effects analysis (FMEA) of the candidate system must be performed manually to determine the effects of component failures on the system. For advanced avionics systems incorporating complex redundancy management schemes, this can involve exploration of system component interrelationships which approaches combinatorial explosion. Using known reliability techniques, it is nearly impossible to completely analyze the reliability of a system before the system has been finalized and implemented. Furthermore, since current reliability models are generated manually, errors may be entered into the evaluation process which may not be discovered until well after the design is finalized.
In modern fault tolerant systems, the interrelationship between components are too complex to model. For example, modern aircraft employ multiprocessor real time computer systems which control the surface of the aircraft in flight based on inputs from sensors. The computer system then generates control laws which are used to control the surface actuators. Reliability of components in prior systems was largely based on experience, wherein block diagrams of components are manually mapped for each individual component. In complex systems, the time required to generate a reliability model often exceeds the allocated time for finalizing a system architecture, as noted above.